This invention relates to dental implants, particularly to implants intended for installation in the maxillary and mandibular posterior regions where bone is cancellous internally and cortical externally.
As it has developed to the present time, the technology of dental implants preferentially employs cylindrical implants, some externally threaded, and some not threaded, but all being much longer than they are wide with the ratio of length to width being about 1.8 to 5.3, for example. This may be due primarily to the fact that early successes were experienced with installation in the anterior area of dental arches. Thus, dental implants commonly available at the present time have lengths ranging up to about 20 mm and widths up to about 4 mm. Predictability of this type of installtion in the anterior area of dental arches is now so good that the use of dental implants has entered the armamentarium of oral surgeons, prosthodontists and periodontists in the treatment of fully and partially edentulous patients. Attempts to install dental implants in posterior regions of the maxillary and mandibular arches have, however, encountered several unique problems.
On the one hand, such attempts have been frustrated by the presence of the inferior alveolar mandibular canal in the posterior mandible, and by the presence of the sinus cavities superior to the posterior maxillary bone. Risk of invading the sinus cavities and the mandibular canal is generally avoided, the result being that often in these posterior regions no more than about 8 mm or less of bone depth is available in which to bore a site to receive a dental implant fixture. Therefore, very short implants were placed with fewer cubic millimeters in bone for foundation. Lekholm reported reduced success with shorter implants. (2nd Int. Tissue International Congress, Rochester, Minnesota, Sept. 1990). In order to place longer implants in these regions, many surgeons have resorted to more different techniques, including sinus lift procedures and mandibular canal repositioning. These procedures are obviously of greater risk than standard implant treatment in the anterior regions of the mouth. It is an advantage of the present invention to avoid these procedures. Some practitioners have sought to overcome this problem in the mandible, if the mandibular canal is located in a buccal position, by installing an available dental implant fixture closer to the lingual surface, and thereby bypassing the mandibular canal, when adequate bone is available to the lingual surface to avoid the risk of fenestration. This procedure, when available, may have the advantage of providing partial primary stabilization in cortical bone, which is important for eventual osseointegration of the fixture with the bone.
It has become apparent that wider jawbones (as in the posterior regions) usually have less trabeculation and often are without adequate amounts of density of bone in their marrow spaces to provide anchorage for dental implants. In the maxillary and mandibular posterior regions the bone is cancellous internally and cortical externally, a condition sometimes termed "eggshell". It has been found to be often almost impossible to securely immobilize a dental implant in the marrow spaces of posterior jawbone regions. Jaffin & Berman noted less success in bone in posterior regions. J. Periodontal, 1991, 62:2-4. It has been suggested that the only hope of more predictable success in these cases is to place a dental implant so as to engage a denser, more cortical layer of bone that often protects the maxillary and nasal sinuses, or that covers the mandibular canal, or engaging buccal-lingual plates in the posterior mandible or maxilla, all of which have inherent surgical risk.
The above-described difficulties and proposed solutions are presented in greater detail in an article by Langer, B. et al entitled "Osseointegration: Its impact on the Interrelationships of Periodontics and Restorative Dentistry: Part 1: The International Journal of Periodontics & Restorative Dentistry, Volume 9, Number 2, 1989, at pages 85 to 105.